Forming dielectric films



' -R. D. MERSHON, FORMING'DIELECTRIC FILMS.

APPLICATION FILED FEB. 18, 1920.

" W0. EVENTOR ln fp g M TTORNEYS Patented Aug. 30, 1921.

a full, clear, and exact description.

UNITED STATES PATENT OFFICE.

RALPH D. MERSHON, OI NEW:YOR K, N. Y. i

, FQRMING DIELECTRIC FILMS- Specification of Letters Patent.

Patented Aug. 30, 1921,

' appltcationflled February 18, 1920. Serial 359,573.

To all whom it. may concern.

Be it known that I, RALPH D. MERSHON, a citizen of the United States ofAmerica. residing at New York, in the county and State ofNew York, haveinvented certain new and useful Improvements in Forming DielectricFilms, of which the following is As is-well known, the action ofelectrolytic condensers, rectifiers and other, devices depends upon thefilm which may be formed upon the surface of aluminum, tantalum andother metals immersed in certain electrolyte and subjected to theelectric current; which film possesses the remarkable property ofallowing current to flow from the electrolyte to the electrode with butlittle or no opposition, but of strongly opposing current tending tofiow from the electrode to the electrolyte. In tending tolpass throughthe'film the purrent exerts a stress thereon which may be convenientlytermed film stress. Flow of current from the electrolyte to theelectrode is accompanied by the formation of minute holes orperforations for its pas-' sage, but when the direction of the currentis reversed these holes will behealed up by re-formation of the filmover them.. The re-formation, however, is at the ex ense of theunderlying metal, as is the origlnalformation of the film, and hencerepeated perforation and re-formation will inthe course of timeseriously corrode the metal and eventually destroy the electrode. Henceit is in general highly desirable to have the film stress constant indirection, that is, tending to send current from the electrode to theelectrolyte, so that perforation will not occur. For this reason the"practice heretofore in formin the films has been to use direct currentor the purpose, the positive pole of a suitable source of such currentbemg connected to the electrodes and the negative pole to theelectrolyte. In subsequent use of the filmed electrodes in a condenseron an alternating current circuit reversal of the film stress isprevented-by exciting the condenser, that is, by providing aunidirectional E. M. F. opposing that between thev electrolyte and theelectrodes, as fully explained in my prior Patent No. 1,077,628, datedNovember 13, 1913. In forming films .with direct current the voltageemployed is usually raised a little above the maximum voltage at whichthe apparatus is to be used on alternating current and the formation:preferably continued at this voltage until the current passed by thefilms has fallen to, a suitably low value. It has been found that thebehavior of the film is as if-its thick-' ness were a direct function ofthe forming voltage, that is, the higher the forming voltage the thickerthe film. But it has also been found that there is a point, which maybeconveniently termed the critical voltage, be-

yond which increase of the forming voltage does not materially increasethe thickness of the film; or, if it does,'it requires an indefinitelylon time for its accomplishment.

' .I- have iscovered, however, that the films may-be formed by thesimultaneous use of both direct and alternating current, even with theapparatus in operation as a condenser during practically the entireperiod of formation, and-it is accordingly the chief object. of mypresent invention to provide a convenient and effective method for thepurpose.-

The drawing illustrates diagrammatically the preferred method ofcarrying out my invention. Y j 1 .In carrying out the inventionin thepreferred manner the condenseris first excited at a low unidirectionalvoltage, thus forming or partially forming the film up to that volt age,and, while the direct voltage is still im- To this and other ends theinvention consists in the novel features hereinafter described.

pressed, alternating voltage of low value is also supplied. Theapplication of the alternating volta e causes an increase of currentpermissible current ca acity of the direct current machine supplymg thedirect current the alternating voltage is raised untilthe currentthrough the D. C. machine has risen to the maximum value permissible forthe machine. The increase of alternating voltage produces further filmformation, with consequent decrease of direct current passed, I

and when the latter has again fallen to a suitably low value thealternating voltage is again raised. This process is continued until thealternating voltage is the maximum that can be emplqy ed withoutreversing the film stress. hen this point is reached the unidirectionalvoltage is increased by a convenient .amount, and when, because offurther film formation, the direct current passed by the films hasfallen sufli ciently the alternating voltage is again increased.

The steps described are repeated as often .as necessary to form thefilms up to the thickness desirable in subsequent operation of thecondenser, or even farther. In fact, if the condenser is intended to beoperated on a voltage at or near the critical value of the film stressthe formation may be carried on' at voltages hi her, even considerablyhi her, than the'critical value.

nstead of increasing the forming voltages.

much smaller direct current generator is required than where theformation is carried up to the maximum desired film stress by means ofdirect current alone, since in the latter case the generator must supplyand Thus in the drawing, the plates carry the current involved by thehigh voltage used. In the present method the direct current machine mustalso carry the forming current, but at a much lower voltage. Secondly,since the condenser is connected with an alternating current circuitduring formation of the films,- it is possible at any stage to determinethe power factor and other constants of the condenser merely byinserting the proper measuring or indicating instruments in the circuitwithout disturbing the internal or gxternal connections of thecondenser. I

In practising the invention in the preferred manner'the electrodes whichare to be filmed are connected to the alternating source as a condenser,that is, half the plates are connected to one terminal of the source andthe otherlhalfto the other terminal. immersed in a suitable electrolyte11 contained in a tank 12, are connected to one terminal ofthe otherterminal. Inea'ch case. theicopnection alternating source-14, throughthe medium of the autotransformer. 15, and the other plates,16,...aregsim1larly connected to the nected to the electrolyte, as bymeans of a I cathode 20, composed of gold, nickel, or other suitablenon-filming material. In starting the formation of the film the variableconnections are set to give the desired l ow voltages and are shifted toincrease the voltages as already described. It is to be understood thatI do not limit myself to any particular method of varying the voltages.In order to prevent reversal of the film stress there is a minimum valueof. the unidirectional voltage at the plates, relative to thealternating voltage, at or above which the former should be keptregardless of absolute values. This minimum value is one-half themaximum instantaneous value of the alternating volta e. Thus, in thecase of an alternating E.

and representin the value of the alternating voltage by%@ (the squareroot of the mean of the squares of the instantaneous values), theunidirectional voltage should,

theoretically, never be less than HE thoughit may be as much higher asdesired.

. F. wave of simple sine form, v

- It is to be understood that the invention is not limited to the exactprocedure herein specifically described but can be practised in otherways Without departure from its spirit. h I I -I claim:

1. In the art of forming dielectric films of the kind described, theimprovement comprising impressing upon the electrodes immers' ed in asuitableelectrolyte both direct and alternating voltages of suchrelative values as to prevent reversal of the film stress.

'2."'In the art of forming dielectric films of the kind described, theimprovement comprising impressing on the electrodes immersed-ina-suitable electrolyte both direct and alternating voltages of suchrelative values as to produce a film stress constantly directed from theelectrodes toward the electrolyte.

3. In the art of forming dielectric films of i the klnd described, theimprovement comprising immersing the electrodes in a suitable electrole, connecting'the electrodes to a source of a te'rnating voltage in themanner of'a condenserand connecting the electrodes to the positive poleand the electrolyte to the ne ative 'pole of a'source of unidirectionalvo tage to impress both unidirectional-and alternating voltages ofsuitable values, and thereafter increasing unidirectional voltages, r

4. In the art of forming dielectric films the alternating and of thekind described, the improvement com- I alternating voltage of a valuetoo low to reprisin' immersing theelectrodes in a suitable e ectrolyte,impressing ,on the electrodes both unidirectional and alternating volt;ages ofsuitable values, and thereafter in creasing the unidirectionaland alternating voltages with such relative valuesas will prevent filmstress from the electrolyte to the electrodes.

5. Inthe art of forming dielectric films of the kind described, theimprovement com.- prising immersing the electrodes in a suitableelectrolyte, impressing unidirectional voltage "on the electrodestopartially form the films, and while supplying unidirectional voltage tothe electrodes, operating the ap paratus as a condenser in connectionwith a source of alternating voltage. v

6. In the artof forming dielectric films of the kind described, theimprovement comprising impressing on the electrodes immersed in asuitable electrolyte a unidirectional voltage of relatively low value tostart formation of the films and exert. a film stress from theelectrodes to the electrolyte,

and Pater impressing on the electrodes an verse the film stress. I

7. In the art of forming dielectric films of the kind described, theimprovement comprising impressing on the electrodes immersed in asuitableelectrolyte a unidirectional voltage of relatively low value tostart formation of the films and exert -a film stress from theelectrodes to the electrolyte, impressing on the electrodes ranalternating voltage of a value too low toreverse the film stress, andwhen the direct current'through the unidirectional source has decreasedto a suitable value increasing the alternating voltage to increase thefilm stress from the electrodes to the electrolyte. 8. In the art offorming dielectric films of the-kind described, the improvement com;

prising impressing .on the electrodes immersed in a suitable electrolytea unidirectional voltage of suitable value to start for- I so mation ofthe films and exert a film stress from the electrodes to theelectrolyte, impressing on the. electrodes an alternating voltage ofavalue high enough-toincrease ..,signature.

without materially exceeding the .permis sible current-carrying capacityof the source of unidirectional voltage, increasing the alternatingvoltage one or more times without reversing the film stress, increasingthe unidirectional Voltage to a safe value, and repeating' one ormore ofthe foregoing steps until the films are formed to the thickness desired.

9. In the art of forming dielectric films of the kind described, theimprovement com-, prising impressing on the electrodes immersed in asuitable electrolyte a unidirectional voltage of relatively low valueand thereby forming, the filmsup to such voltage, impressing arelativelylOW alternating voltage on the "electrodes until the resulting increasedcurrent through the films "has fallen to a suitable value, andthereafter increasing the alternating and unidirectional voltageswithout causing flow of current fromvthe electrolyte to the electrodesand without materially exceeding. the current. carrying capacity of thesource of unidirectional voltage.

10. In the art of forming dielectric films prisingsubjecting theelectrodes immersed in a suitable electrolyte to unidirectional andalternating voltages, and progressively increasing such voltages'withoutcausing a film stress from the electrolyte to the electrodes and withoutmaterially exceeding the ofthe kind described, the improvementcompermissiblecurrent-carrying capacity of the source of unidirectionalvoltage, until the films are formed up to.at least the maximum voltageatv which the electrodes are to housed in an electrolytic condenser.

11, In the art of forming dielectric films of the kind described, theimprovement comprising impressing on the electrodes imrectional andalternating voltages, and pro.- gressively increasing such voltages atsuitmersed in a suitable electrolyte both unidiable rates until thecritical film stress is exceeded.

Intestimony whereof I-hereunto aflix my RALP H 15. MERSHON.

